A typical semiconductor CMOS (complementary metal oxide semiconductor) inverter, as is conventionally known is shown in FIGS. 1A and 1B. FIG. 1B is a cross sectional view of FIG. 1A and FIG. 2 shows an equivalent circuit of the CMOS inverter of FIGS. 1A and 1B.
As is well known, the CMOS inverter comprises an N MOSFET (metal oxide semiconductor field effect transistor) and P MOSFET.
Referring to prior art FIGS. 1A and 1B, the structure of an N MOSFET is shown including a P type well region 2 formed on an N type semiconductor substrate 1. A source region 3 and a drain region 4 are formed in the P type well region 2. The source region 3 and the drain region 4 are N+ type impurity diffusion regions. Moreover, a gate insulation film 5 and a gate electrode 6, which is formed of polycrystalline silicon, are formed on the semiconductor substrate 1.
Next the structure of the P MOSFET will be explained.
The P MOSFET is formed in the N type semiconductor substrate 1. The P MOSFET is isolated from the N MOSFET by a field oxide film 7 and an isolation region 8. A source region 9 and a drain region 10 are formed in the semiconductor substrate 1. The source region 9 and the drain region 10 are P+ type impurity diffusion regions. Moreover, a gate insulation film 11 and a gate electrode 12, which is formed of polycrystalline silicon, are formed on the semiconductor substrate 1. Gate electrode 12 may be interconnected to gate electrode 6 of FIG. 1B. Furthermore, the N MOSFET and P MOSFET are coated with a silicon oxide film 13. A contact hole 14 is formed by removing a portion of the silicon oxide film 13. The drain regions 4 and 10 are electrically connected to a metallic wiring layer 15 preferably of aluminum at the contact hole 14.
Copper or silicon may be added to the aluminum wiring layer 15. Since the atomic radius of copper is large, the copper has a technical advantage in which electromigration of the aluminum wiring layer 15 can be prevented. Also, at a contact portion between the semiconductor substrate 1 and the aluminum wiring layer 15, the silicon is used to control the diffusion of silicon of the semiconductor substrate 1 into the aluminum wiring layer 15 and to prevent the shape of the surface of the contact portion from being changed. Following conventional practice, the aluminum wiring layer 15 to which copper or silicon may be added is then annealed. If the aluminum wiring layer 15 is annealed so that the diameter of crystal grain becomes larger, resistance to electromigration is improved. As a result, the life time of the aluminum wiring layer 15 is prolonged.
In recent years, as shown in FIG. 1B, such an annealing process using a laser beam has been tried. According to such method, the laser beam 16 is directly irradiated onto the aluminum wiring layer 15 which is heated up to close to 660.degree. C., which is the melting point of aluminum. Thereafter, if the aluminum wiring layer is recrystallized, the diameter of crystal grain becomes large. This increases the life time of the aluminum wiring layer 15.
However, aluminum reflects most of the energy of the laser beam. Actually, 90% or more of the energy density (J/cm.sup.2) of the laser beam 16 is reflected from the surface of the aluminum wiring layer 15 at the time of annealing. Since energy density of the laser beam 16 corresponds to quantity of heat, 90% or more of the entire quantity of heat of the laser beam is lost. As a result, to provide for sufficient annealing, the energy density of the laser beam must be increased to a level substantially above 0.5 J/cm.sup.2 and up to about 0.7 (J/cm.sup.2) .
As apparent from FIG. 1B, the laser beam 16 is also being irradiated onto the silicon oxide film 13, which is not coated with the aluminum wiring layer 15. As a result of such high density radiation from the laser beam, the gate electrodes 6 and 12, which are formed of polycrystalline silicon, are unfavorably influenced. If the energy density of the laser beam is high enough, the gate electrodes 6 and 12, may completely melt and disappear. Even if not melted, reliability of the gate electrodes 6 and 12 is reduced, and in the worst case, the electrical connection is broken.
As mentioned above, although the aluminum wiring layer can be fully annealed by a laser beam having a high energy density, this exerts an unfavorable influence on the gate electrodes formed of polycrystalline silicon. Conversely, the aluminum wiring layer 15 cannot be fully annealed by a laser beam having low energy density.